I meant to write: "I am aware that Message-Ids are, in theory, unique,
but in practice, non-unique."
Ah.
Of course that, two different message contents with the same Message-ID, is a much worse problem if they happen to arrive at the same Inbox. Then the email interface (especially if it is Gmail) may choose not to show one of them (the "duplicate" message).
At least the problem Mark was solving is more benign - the same message content arriving with two different Message-IDs merely results in the user seeing both copies in his/her inbox.